Fixing an HP 5342A microwave counter. Part 2.

Continuing from Part 1.
I got a new CPU, this time in a plastic package, not a shiny ceramic with golden cap, but should be good otherwise. The questions are: a) was the original CPU faulty and b) did a new one fix the instrument?

And the answers are Yes and No.

I installed the replacement CPU and tested. Exactly the same result, the instrument is frozen with garbled front panel. However, I don't have the glitches on the address lines. While apparently this did not affect anything, it still indicates that something is at least marginal with the old CPU.

How to proceed? At one time HP was very keen on signature analysis, which can indeed help in some cases at least by confirming that the tested part of the circuit works.

I am now beginning to suspect that one of the ROMs may be bad, which would suck, but first I must make sure that I can test is properly.

Why do I think that it may be ROM problem and not, for example front panel? Using the test switches on the CPU board, I can disable individual bits of the data bus. Obviously the CPU would be running a nonsense random program in this case, but for example disabling bit 3 will make it run in some loop, where the front panel display are changing. Obviously, the strobe signal to write into the panel is activated randomly and whatever random data is there will get written into the display.

At least this is what I think, it is still possible that the front panel is faulty and I'm chasing a red herring on the CPU board. But first I want to confirm whether the CPU board is good or not.

For this there are extensive signature tests in the service manual. First in free run (NOP) mode, which tests all the address lines and all the various device select signals which actually covers a large part of the login on the board. This can indeed be done by powering up the CPU board outside the instrument.

So, let dig out the trusty old Sony/Tektronix 308 Data Analyzer, which can do signature analysis and some rudimentary serial and parallel logic analysis and looks like a cute little oscilloscope. And for some insane reason it is usually cheaper on eBay than a HP signature analyzer which can only do signature analysis.

Well, all signatures check out OK. So at least these parts are good.

The next step would be to do a signature test on the ROMs. And that is where I stop now, as it does require the CPU extender card which contains additional logic to provide the start and stop signals for the ROM signature test. I don't have this card, but a kind gentleman on the Vintage HP equipment group pointed out that the schematics for the extender is in fact in the service manual.

It is not complicated, so I'm now tempted to put together one on a breadboard and be able to do the analysis as I really want to confirm if the ROMs work or not.
So I'll continue in Part 3.